1. Field of the Invention
The present invention relates to a digital broadcast receiving apparatus, and more specifically, to a digital broadcast receiving apparatus for receiving a digital television broadcast.
2. Description of the Background Art
In recent years, the digitization project of television broadcast is rapidly progressing worldwide. Particularly, in the field of digital television broadcast, the use of MPEG2 (Moving Picture Experts Group 2), which is an international standard of data compression/decompression, enables transmission of a plurality of channels with a single repeater so that the number of channels can be significantly increased. As a result, convenience for the user is expected to improve significantly.
In receiving digital television broadcast, there is a need to decode compressed coded data to obtain the actual image signal and audio signal.
FIG. 11 is a schematic block diagram representing an arrangement of a general digital television broadcast receiving system.
Referring to FIG. 11, a digital television broadcast receiving system 1 includes an antenna 2 for receiving RF signals which are airwaves, and a detection-demodulation unit 3 for performing detection and demodulation of the RF signals according to a channel selection instruction from the user. Detection-demodulation unit 3 outputs transport stream data (hereinafter, also simply referred to as TS data) according to the MPEG2 standard. The TS data is formed as one data stream into which coded data respectively corresponding to an image signal, an audio signal, and a data signal are multiplexed.
Digital television broadcast receiving system 1 further includes a TS separating unit 4 for separating the multiplexed TS data, an audio decode unit 5 for decoding audio coded data received from TS separating unit 4, and a video decode unit 6 for decoding image coded data received from TS separating unit 4.
Audio decode unit 5 and video decode unit 6 executes decoding of the compressed coded data according to the MPEG2 standard. An audio signal and an image signal respectively are output from audio decode unit 5 and video decode unit 6 and transmitted to an audio output unit 7 and an image display unit 8. As a result, from the RF signals received by antenna 2, the image signal and the audio signal corresponding to the user channel selection can be output to image display unit 8 and audio output unit 7.
In addition, although not shown in FIG. 11, data signal that is included in the TS data can also be output to image display unit 8 by executing decoding based on the MPEG2 standard.
In the digital television broadcast, however, it is inherently difficult immediately to display a real time image corresponding to the airwaves upon power-on of the receiving apparatus or upon switching of the channel selection by the user (hereinafter, also described simply as “upon channel switching”). During this period, no image is displayed on image display unit 8 so that there is a possibility that unnecessary anxiety and stress may be caused to the user.
Returning to FIG. 11, in order to start data demodulation in detection-demodulation unit 3, it is required that the received data of at least a prescribed amount that is the minimum required for execution of error correction be accumulated. Consequently, upon power-on or upon channel switching, outputting of TS data from detection-demodulation unit 3 is delayed for the period during which the prescribed amount of received data is accumulated. In addition, in decoding the image coded data, there is a possibility of delay being generated in image display due to the nature of the MPEG2 standard.
FIG. 12 is a schematic diagram related to the description of a composition of image coded data according to MPEG2.
Referring to FIG. 12, the image coded data according to MPEG2 (hereinafter, also simply referred to as MPEG2 image coded data) is expressed as bit stream data having a hierarchical arrangement. Particularly, in the MPEG2 standard, data corresponding to several pictures are treated as a prescribed segment called GOP (Group of Pictures), and the GOP structure is adopted which allows random access by the unit of GOP.
In other words, MPEG2 image coded data is formed by a plurality of GOPs, and a sequence header SH is provided corresponding to each GOP. Information such as data size is recorded in the sequence header SH, and the sequence header SH is used, for instance, to locate the start of random access performed by the unit of GOP.
One GOP includes picture data generated by compression-coding a prescribed number of original pictures. In the MPEG2 standard, the picture data can be categorized into the following three picture types based on the compression coding scheme of the original pictures: an I picture (Intra-coded picture), a P picture (Predictive-coded picture), and a B picture (Bidirectionally predictive-coded picture).
The I picture, also referred to as an intra-coded picture, involves coding of one original picture alone whose information is independent of other original pictures. Thus, the I picture is independent of other pictures that precede or succeed it and can be decoded by itself. Each GOP is formed such that it includes at least one I picture.
The P picture, also referred to as an inter-frame predictive-coded picture, utilizes an I picture or a P picture of an earlier time already decoded as a predicted image (image that serves as a reference in order to take the difference) and involves coding of the motion-compensated difference between the original P picture and the predicted image.
The B picture, also referred to as a bidirectionally predictive-coded picture, is coded as an image to be inserted between the I picture and the P picture after the I picture and the P picture are processed in advance. In other words, the B picture utilizes the following three kinds of pictures as a predicted image: 1) the I picture or the P picture of an earlier time already decoded, 2) the I picture or the P picture of a later time already decoded, and 3) an interpolated image made from 1) and 2).
Thus, video decode unit 6 cannot output an image signal until a sequence header and an I picture is properly decoded. Moreover, in order to adjust the output timing of audio decode unit 5 and video decode unit 6 so as to ensure the synchronization between the image signal output to image display unit 8 and the audio signal output to audio output unit 7, i.e. the AV synchronization, a certain amount of coded data must be accumulated in audio decode unit 5 and video decode unit 6. As a consequence, image display would also be delayed for the period during which coded data required to ensure the AV synchronization is accumulated. The delay of the image display upon power-on or upon channel switching due to these factors may be expected to amount to about two to three seconds.
On the other hand, as a user is selecting a channel, when a specific channel that the user wishes to view cannot be found, the user tends to manipulate the channel selection by selecting the channels instantly and continuously in order to grasp in a short time the contents of broadcast on the respective channels. Hereinafter, such channel selecting manipulation would also be referred to as a “channel sampling manipulation.”
It is likely that the “channel sampling manipulation” that the user routinely performs with the analog television broadcast cannot be performed smoothly with the digital television broadcast due to the influence of delay in the image display upon channel switching described above. Particularly, after the user inputs a channel switching instruction, a period created during which no image signal is output by image display unit 8 is very likely to cause the user discomfort due to unnecessary anxiety and stress.